Scientist from the University of Cambridge have found that the performance of novel thin film solar cells based on lead-halide perovskite could be further improved, since different parts of these films show different properties. The team lead by Dr. Felix Deschler studied the light emission from thin films of these materials with high spatial resolution and observed spatial inhomogeneity and localized regions of high emission with dimensions of ~500 nm.
“In our study we have used high resolution technique, scanning near-field optical microscopy (SNOM), which enabled us to observe those local changes”, said Milan Vrucinic – first author of the study and Winton Scholar at the Cavendish Laboratory, “We were able to observe regions of increased emission emitting light with a narrower energy range, which we interpret as areas of enhanced structural order and potentially increased crystallinity.”
These results provides a strategy to improve the properties of devices, such as solar cells and LEDs, by expanding the observed high quality regions to larger areas of the film. The results are published in Advanced Science (http://onlinelibrary.wiley.com/doi/10.1002/advs.201500136/abstract).
Perovskite materials have shown great promise in photovoltaic applications with power conversion efficiencies recently exceeding 20%. The excellent semiconducting properties of these solution-processed materials lead to the fabrication of bright light emitting devices (LEDs) (http://www.nature.com/nnano/journal/v9/n9/full/nnano.2014.149.html) and the demonstration of optically pumped lasing (http://pubs.acs.org/doi/abs/10.1021/jz5005285), which show the potential use of lead-halide perovskites in a wide range of optoelectronic devices.
“While spatial PL intensity variations might be generally expected in a polycrystalline film, it is surprising that these can be as localized as in our observations. These results suggest that excitations in high quality regions are not affected by more defective regions next to them, which could be one of the reasons why these materials show such good performance.”, said Dr. Felix Deschler, Herchel Smith Research Fellow at the University of Cambridge.
A critical process for optoelectronic performance is the diffusion of charge carriers, which affects charge extraction and radiative recombination. In particular it is crucial to understand the differences between the length scales governing charge extraction, which is important for solar cells, and charge recombination, which applies to LEDs. The recent study from the Cavendish finds that radiative recombination seems to be governed by shorter length scales than the ones previously extracted from charge extraction measurements, which will help to fabricate more efficient LEDs in the future.